Process for casting molten silicon-aluminum killed steel continuously

ABSTRACT

A molten silicon-aluminum killed steel consisting essentially of 0.04-0.30% of carbon, 0.20-1.50% of manganese, 0.02-0.15% of silicon, 0.04-0.10% of aluminum, and the remainder iron and incidental impurities is introduced into a mold for continuously casting steel and is cast therein to produce a solidified steel shape continuously. The molten steel is introduced into the mold through a tundish nozzle having a size sufficiently large to prevent plugging by nonmetallic inclusions. The solidified steel shapes may be cast in the form of slabs which may be hot rolled to produce flat rolled products such as steel strip or plate. In one preferred variant, the quality of the solidified steel shapes is improved by preventing oxidation of the aluminum content of the molten killed steel prior to casting, by providing a fluid slag over the molten steel in the mold to aid in separating nonmetallic inclusions therefrom prior to solidification, and/or by agitating the molten killed steel in the ladle with an inert gas to reduce nonmetallic inclusions and assure uniformity of composition and temperature.

United States Patent [191 Miltenberger et al.

[5 PROCESS FOR CASTING MOLTEN SILICON-ALUMINUM KILLED STEEL CONTINUOUSLY [75] Inventors: Robert S. Miltenberger, Weirton,

W. Va.; David W. Wilcher, Winterville, Ohio [73] Assignee: National Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 16, 1972 [21] Appl. No.: 263,626

Related US. Application Data [63] Continuation-impart of Ser. No. 841,134, July 11,

1969, abandoned.

ADDITIONS r 1 [111 3,822,735 1451. July 9,1974

917,786 2/1963 Great Britain 164/82 OTHER PUBLICATIONS Journal of Metals, April 1968, TN1J6. pp 89-94.

Metal Progress, July 1971, TS300.M587. pp. 8889.

Primary Examiner-R. Spencer Annear Attorney, Agent, or FirmShanley and ONeil 57] ABSTRACT A molten silicon-aluminum killed steel consisting essentially of 0.040.30% of carbon, 0.201.50% of manganese, 0.02-0.15% of silicon, 0.04-0. 10% of aluminum, and the remainder iron and incidental impurities is introduced into a mold for continuously casting steel and is cast therein to produce a solidified steel shape continuously. The molten steel is introduced into the mold through a tundish nozzle having a size sufficiently large to prevent plugging by nonmetallic inclusions. The solidified steel shapes may be cast in the form of slabs which may be hot rolled to produce fiat rolled products such as steel strip or plate. In one preferred variant, the quality of the solidified steel shapes is improved by preventing oxidation of the aluminum content of the molten killed steel prior to cast-v ing, by providing a fluid slag over the molten steel in the mold to aid in separating nonmetallic inclusions therefrom prior to solidification, and/or by agitating the molten killed steel in the ladle with an inert gas to reduce nonmetallic inclusions and assure uniformity of composition and temperature.

20 Claims, 1 Drawing Figure IIERTGAS minnow PATENTED JUL 91974 3. 822 T 3 5 LADLE INERT GAS ADDITIONS AGITATION u u L LADLE O TUNDISH 0 LADLE CASTING- PINCH (SUPPORTING) ROLLS 2! SLABS HOT ROLL COLD ROLL PROCESS FOR CASTING MOLTEN SILICON-ALUMINUM KILLED STEEL CONTINUOUSLY RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 841,134, filed July 11, 1969, on behalf of Robert S. Miltenberger and David W. Wilcher for Process For Casting Molten Silicon-Aluminum Killed Steel Continuously, now abandoned.

BACKGROUND OF THE INVENTION This invention broadly relates to a process for casting molten killed steel continuously. In some of the more specific variants, the invention further relates to a novel process for casting silicon-aluminum killed steel continuously to produce solidified steel shapes which may be rolled into products such as steel strip and plate. a

One recent important innovation in steel making is the casting of molten steel continuously into semifinished shapes such as blooms, slabs and billets. The successful continuous casting of shapes equivalent in section to conventional semifinished shapes eliminates the ingot and primary mill stages of conventional prior art rolled steel production, and thus offers important economic advantages.

Although the continuous casting of steel seems to be simple in principle, there are many difficulties inherent in the process. This is due in part to the high melting point, high specific heat and low thermal conductivity of steel, and the necessity for close control of variables such as the temperature and oxygen content of the molten steel. The molten steel must be killed sufficiently to prevent pin holes from forming in the surface of the solidified shapes as they are cast. Any tendency of the molten steel to effervesce excessively and form pin holes or blowholes in the thin solidified skin that is formed initially around the molten steel interior of the shapes is very undesirable. This is true from the standpoint of weakening the skin and increasing the chance of escape of molten steel therefrom, as well as from the standpoint of surface imperfections. The initial temperature of the molten steel fed to the mold from the tundish via the tundish nozzle is of importance as the solidification rate must be predictable so as to form a skin of sufficient thickness and strength to support the casting. It is therefore apparent that a successful continuous casting operation depends to a large extent upon providing properly deoxidized molten steel which is at a uniform casting temperature.

The melt of steel to be deoxidized and continuously cast may be prepared by a number of prior art steel making practices, but the basic oxygen process results in substantially lower costs and is often preferred. In accordance with one prior art basic oxygen process, the furnace is charged with scrap, molten ferrous metal from a blast furnace, and other charge materials necessary to produce a low carbon steel upon blowing with oxygen. In instances where the melt is to be cast into conventional ingots, the final temperature need not be higher than about 2,90QF. and it is possible to stop the blow after the manganese, silicon, phosphorus and sulfur have been reduced to desirable levels and sufficient carbon is present to impart strength to the steel after vacuum degassing. In instances where the melt is to be cast continuously into semi-finished shapes, the melt temperature should be about 50-75F. higher, i.e., about 2,9502,975F. and it is necessary to continue the oxygen blow for a longer period of time to assure that the higher melt temperature is attained. This usually lowers the carbon content to about 0.040.06% and the oxygen content is about 400-700 parts per million. It is necessary to deoxidize the steel before continuous casting and in theory deoxidation may be effected by making aluminum and/or silicon additions. However, it is not possible to continuously cast a low carbon steel which contains more than 0.020.03% aluminum by the prior art processes. the steels to be continuously cast have been limited heretofore to silicon killed steel or steels deoxidized by vacuum degassing. When attempts were made to continuously cast siliconaluminum killed steel having aluminum contents above 0.02-0.03% in accordance with the prior art practices, the nonmetallic inclusions in the steel deposited on the internal surface of the tundish nozzle choked off the flow of molten steel.

It is essential that the molten steel be fed to the continuous casting mold in a predictable manner and so as to maintain a controlled level of molten metal, and thus the deposition of refractory nonmetallic inclusions in the tundish nozzle cannot be tolerated in a continuous casting operation. As a result, silicon-aluminum killed steels containing more than 0.02-0.03% of aluminum were not thought to be capable of being continuously cast heretofore.

The presence of aluminum in the molten siliconaluminum killed steel in amounts over 0.020.03% also tends to cause large quantities of nonmetallic inclusions to accumulate along the surfaces of continuously formed castings produced by prior art processes. As a result, castings of inferior quality are produced which require a large amount of surface conditioning before they are suitable for rolling into flat rolled steel products. Aluminum oxide containing nonmetallic inclusions are especially undesirable when continuously casting slabs to be rolled into steel strip and plate as massive agglomerates form along the side walls of the mold as the partially solidified steel casting descends. The agglomerates are difficult to remove and mar a substantial portion of the surface area of the fully solidified slabs.

As a result of the foregoing and other limitations and disadvantages of the prior art, an entirely satisfactory process for continuously casting silicon-aluminum killed steels having an aluminum content above 0.020.03% was not available prior to the present invention. Inasmuch as there is a large demand for flat rolled silicon-aluminum killed steel products for uses which require good hardness, toughness, drawability, formability, stability, and ductility, it would be very advantageous to provide a continuous casting process for silicon-aluminum killed steels as yields over conven tional ingots would be markedly higher and production 7 costs substantially lower. I

It is an object of the present invention to provide a novel process for casting silicon-aluminum killed carbon steels continuously.

It is a further object to provide a novel process for preparing molten silicon-aluminum killed steel from a melt of steel produced by a basic oxygen process, and then continuously casting the molten steel into soliditied steel shapes.

It is still a further object to provide a novel process for continuously casting slabs from silicon-aluminum killed steel which may be rolled to produce flat rolled products such as steel strip or plate.

It is still a further object to provide a novel process for continuously casting silicon-aluminum killed steel wherein the quality of the solidified steel shapes is improved.

Still other objects and advantages of the invention will be apparent from the following detailed description and the examples.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE of the drawing schematically illustrates one suitable arrangement. of apparatus for continuously casting steel in accordance with the pro cess of the invention.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF In accordance with one important variant of the pres ent invention, a molten silicon-aluminum killed steel consisting essentially of 0.040.30% of carbon, 0.20l.50% of manganese, 0.02-0.l5% of silicon, 0.040.10% of aluminum and the remainder iron and incidental impurities is introduced into a mold for continuously casting steel and is cast therein to produce a solidified steel shape continuously. The phosphorus content of the steel should be less than 0.05%, the sulfur content should be less than 0.05%, and the oxygen content should be less than I25 parts per million. Pref erably, the steel contains 0.040.30% of carbon, 0.20l.50% of manganese, about 0.060.08% of silicon, about 0.040.05% of aluminum, about 0.0050.02% of phosphorus, about 0.0l0.035% of sulfur, and less than 100 parts per million of oxygen. For best results, the steel should contain about 0.0050.0l5% of phosphorus, about 0.0l80.026% of sulfur, and less than 75 parts per million of oxygen. As will be described more fully hereinafter, there are certain preferred compositions, procedures and conditions which produce superior results.

A melt of steel for use in practicing the present invention may be obtained from any suitable prior art steel making furnace such as an open hearth furnace, an electric furnace, or a basic oxygen furnace. However, it is usually preferred to use molten steel produced by a basic oxygen process.

The prior art basic oxygen process for producing low carbon steel is satisfactory without modification. The low carbon steels produced by a basic oxygen process usually contain not more than 0.30% and often less than 0.l5% of carbon, and in many instances about 0.060.l2% of carbon. The manganese content of the steel is less than 1.0% and is usually about 0. l00.40%, the silicon content is less than 0.02% and is usually below 0.0l%, the phosphorus content is less than 0.04% and is usually below 0.02%, the sulfur content is less than 0.05% and is usually below 0.03%, and the remainder is iron and incidental impurities. Trace amounts of tramp elements such as copper, tin, lead, zinc and the like may be present, but their concentrations are very low. The oxygen content of the steel is below 1,000 parts per million (ppm) and is usually below 800 ppm, and is often about 400700 ppm. The temperature of the melt is usually about 2,8503,000F., and is preferably 2,9lO-2,970F. for continuous casting.

Basic oxygen steel making processes and apparatus and continuous casting processes and apparatus for use in preparing steel melts and continuously casting the same are disclosed in numerous patents and literature references, including the text The Making, Shaping and Treating of Steel, 8th Edition, edited by Harold E. McGannon (1964), the disclosures of which areincorporated herein by reference. Pages 453456 and 664666 of the text The Making, Shaping and Treating of Steel" are especially pertinent.

It is not possible to produce a furnace melt having the desired composition for continuous casting as tapped. Accordingly, it is understood that the melt of steel initially produced by a prior art basic oxygen process or other suitable steel making process is adjusted in composition to produce a silicon-aluminum killed steel as defined herein. This may be conveniently accomplished by charging the steel making furnace with ingredients and employing operating procedures and practices which will result in approximately the desired carbon, phosphorus and sulfur contents in the steel as tapped from the furnace, or which will result in the desired contents thereof after the silicon, aluminum and- /or manganese containing addition agents are added. The silicon and aluminum contents in the steel as tapped are markedly lower than desired, and the oxygen content is markedly higher. It is necessary to add silicon and/or aluminum as necessary to arrive at the desired silicon and aluminum contents in the final silicon-aluminum killed steel to be continuously cast. It is also usually necessary to make manganese additions as it is not practical to charge enough manganese to a basic oxygen furnace to result in the desired manganese content in the steel as tapped due to losses of manganese to the furnace slag.

The silicon, aluminum, and the manganese additions are normally made in the ladle at the time of tapping the heat. The silicon addition may be metallic silicon, or an alloy thereof such as ferrosilicon, silicomanganese, Calsibar (calcium-silicon-barium alloy), or other prior art silicon addition agents. The aluminum addition is preferably made in the form of metallic aluminum, such as pig aluminum or aluminum shot. Manganese may be added in the form of silicomanganese, high or medium carbon ferromanganese, electrolytic manganese, or other suitable alloys of manganese. In instances where it is desired to adjust the carbon content of the steel, this may be done by selecting a high carbon silicon and/or manganese addition agent, or by making a carbon addition in the ladle in accordance with prior art practices.

With reference to the drawing, a heat of low carbon steel having the aforementioned composition may be prepared in a basic oxygen furance (BOF) and then the heat is tapped into a ladle. Ladle additions as aforementioned, including silicon, aluminum and manganese, are made to arrive at a heat of steel having the desired composition. Thereafter the heat of steel may be agitated by introducing therein an inert gas such as argon.

After making the additions to the ladle to arrive at the desired molten silicon-aluminum killed steel composition, the slag covered ladle may be taken to a continuous caster 10 where the steel is transferred therefrom periodically to the slag covered tundish l1, and is about then transferred from the tundish 11 via the tundish nozzle 12 and introduced into the open end of the continuous casting mold 14. The continuous casting mold is water cooled by water provided via conduits l5 and 16, and solidification of the steel is initiated therein following prior art practices. A body of molten metal 17 is present in the top of the mold, and a steel casting 18 with a solidified skin surrounding a liquid metal core is withdrawn from the bottom. Solidification of the molten interior of the casting is accomplished by means of water sprays 19 located below the mold. The solidified steel shape 20, which may be in the form of a slab, may be supported by pinch rolls 21 and withdrawn thereby from mold 17 at a controlled rate. Thereafter the solidification steel shape may be cut into slabs of desired lengths, cooled, surface conditioned reheated to a hot working temperature and hot worked such as by hot rolling following conventional practices to produce flat rolled steel products such as strip and plate. The hot rolled strip may be further reduced in thickness by cold rolling following conventional practices, or it may be given other conventional treatments to arrive at a final steel product.

In accordance with a preferred variant of the invention, the quality of the solidified steel shapes such as slabs is improved by preventing oxidation of the aluminum content of the molten silicon-aluminum killed steel prior to casting, and/or by providing a fluid slag over the molten steel in the mold to aid in separating nonmetallic inclusions therefrom prior to solidification. The oxidation of the aluminum content of the steel may be prevented by providing a fluid slag layer thereover at all times, such as in the tapping ladle, the tundish of the continuous caster, and the open top of the continuous casting mold. The slags that are employed may be in accordance with prior art practices. A lime-silica slag may be employed over the molten metal in the continuous casting mold. A lime-silica slag is effective in agglomerating aluminum oxide-containing nonmetallic inclusions and retaining the agglomerates as they float upward in the molten metal in the continuous casting mold due to their lower specific gravity. The mold usually is reciprocating vertically, and the aluminum oxide-containing agglomerates work upward in the molten metal and are retained in the slag, and are prevented from depositing within the solidified casting or on the surface thereof. The incoming molten metal flowing from the nozzle of the tundish is preferably introduced beneath the slag layer and so as to be in intimate contact therewith. The continuous reciprocation of the mold and the introduction of the molten metal therein beneath the slag layer are very effective in causing the aluminum oxidecontaining nonmetallic inclusions to form massive agglomerates which become trapped in the slag, and the agglomerates are prevented from being carried downward in the mold.

It is also possible to remove aluminum oxidecontaining nonmetallic inclusions from the siliconaluminum killed steel while in the tapping ladle following prior art practices. For example, the steel melt may be agitated in accordance with prior art practices while in the ladle so as to result in successive portions thereof coming into intimate contact with the slag layer. The aluminum oxide forms agglomerates which are attracted to and become incorporated with or entrapped in the slag layer. A number of different methods may be employed for agitating the killed steel, such as by injecting an inert gas into the melt of steel at a point slightly above the bottom of the ladle. The inert gas rises to the surface and expands as the pressure thereon is decreased, and thereby produces a vigorous boiling action which agitates the molten metal. A number of inert gases may be employed such as helium, neon, xeon, and argon, but argon is usually preferred due to its low solubility in molten steel and its relatively low cost. In instances where the killed steel is agitated with an inert gas, the agitation may be from 1 to 15 minutes or longer, but usually is approximately 16 minutes. It is understood that the agitation is terminated when the temperature of the molten steel has reached the casting temperature so as to avoid over-cooling. The agitation also improves uniformity, and assures that the temperature and composition of the steel are substantially the same throughout the ladle.

It is essential that the nozzle leading from the tundish for introducing the molten killed steel into the continuous casting mold have an internal diameter which is sufficiently large to prevent plugging thereof by nonmetallic inclusions while continuously casting the melt of silicon-aluminum killed steel in the ladle. The minimum internal diameter varies directly with the aluminum content in the steel within the ranges set out herein. For example, the minimum internal diameter for casting a silicon-aluminum killed steel having an aluminum content of 0.04% should be about 1% inches, and in instances where the aluminum content is about 0.10%, then the minimum internal diameter should be about 1% inches. These minimum internal diameters are substantially greater than those employed heretofore in continuous casting. For some reason which is not fully understood at the present time, the larger diameter tundish nozzles do not become plugged or choked with the refractory aluminum oxide containing inclusions which are present in the molten steel initially in finely divided form and thus should readily pass through the prior art tundish nozzles. As a result, it is possible to continuously cast the siliconaluminum killed steels of the present invention without interruption over a normal continuous casting cycle. It is understood that the internal diameter of the tundish nozzle may be increased above 1% inches if desired, and the nozzle may be, for example, 2, 3 or 4 inches in diameter.

The composition of the silicon-aluminum killed steel employed in practicing the invention is based upon the results of spectographic analysis of samples. Thus, the total amounts of the various elements in the steel are determined and the steel composition is based thereon.

The foregoing detailed description and the following specific examples are for purposes of illustration only, and modifications may be made therein without departing from the invention.

EXAMPLE I The ladle additions are 650 pounds of 75% ferrosilicon, 950 pounds of pig aluminum, and 1,800 pounds of ferromanganese, and the silicon, aluminum and manganese contents are 0.07%, 0.05% and 0.35% respectively, in the resulting silicon-aluminum killed steel. The oxygen content of the killed steel is approximately 100 parts per million and the remaining constituents in the steel do not change appreciably.

The molten killed steel is stirred in the ladle by inserting a refractory lined pipe therein until the lower end is near the bottom, and injecting argon for approximately 3 minutes. The argon is injected at a rate which results in a vigorous boiling and stirring action. At the end of the argon stirring, the temperature of the steel is 2,9lF., and the ladle is transferred to the continuous caster. The ladle is provided with a prior art slag layer at all times following tapping to prevent oxidation of the aluminum content of the killed steel.

The molten silicon-aluminum killed steel is poured periodically from the ladle into the slag covered tundish of the continuous caster as required to maintain the desired level of molten steel therein. The fluid slag covering the molten killed steel in the tundish prevents oxidation of the aluminum content.

The molten steel is withdrawn from the tundish via the tundish nozzle and is introduced into the upper end of an open-ended water cooled continuous casting mold for casting slabs at a rate to maintain the desired level of molten metal therein. A layer of a fluid prior art lime-silica-fluorospar slag is maintained over the top of the molten metal in the mold to prevent oxidation of the aluminum content thereof and to aid in removing massive agglomerates of aluminum oxidecontaining inclusions. The mold is reciprocated vertically and a casting in the form of a slab having a solidified outer shell and a molten interior is withdrawn from the mold continuously. The casting is sprayed with water as it descends below the mold until it is completely solidified, and it is then cut into predetermined lengths. The solidified casting has substantially the same composition as noted above for the molten killed steel. The slabs are allowed to cool, inspected for surface imperfections, scarfed, reheated to a hot rolling temperature, hot rolled into steel strip having a thickness of approximately 0.1 inch, and then cold rolled to approximately 60% reduction following conventional practices.

The tundish nozzle has an internal diameter of 1% inches and no difficulty is experienced with respect to plugging or choking of the nozzle during the continuous casting cycle of approximately one hour. The slabs require only a routine amount of scarfing to condition them for rolling. Thus, the prior art problems of plugging or choking of the tundish nozzle and poor surface which are characteristic of prior art attempts to continuously cast silicon-aluminum killed steels are overcome.

EXAMPLE ll The general procedure of EXAMPLE I is repeated in another continuous casting run with the exception of employing a tundish nozzle having an internal diameter of seven-eighths inch, which is characteristic of prior art practice. After a short period of time, the tundish nozzle is choked and plugged with aluminum oxidecontaining refractory material. Thus, it is not possible to continuously cast the silicon-aluminum killed steel of the invention under the conditions of this example.

EXAMPLE III The general procedure of Example 1 is repeated with the exception of employing a tundish nozzle having an internal diameter of 1% inches and adjusting the silicon-aluminum killed steel composition to an aluminum content of 0.04%. The tundish nozzle does not plug or choke during the continuous casting cycle and the results are substantially the same as in Example I.

EXAMPLE IV The general procedure of Example I is followed with the exception of adjusting the aluminum content of the molten steel to 0.10% aluminum. No difficulty is expe rienced in continuously casting the steel with the higher aluminum content of 0.10%, and the results are substantially the same as in Example 1.

We claim:

1. A process for casting steel continuously consisting essentially of the steps of preparing a heat of oxygencontaining low carbon steel, deoxidizing the said heat of low carbon steel sufficiently to allow the continuous casting thereof by adding thereto a combination of at least one silicon-containing deoxidizing agent and at least one aluminum containing deoxidizing agent to produce molten silicon-aluminum killed steel, the resulting heat of molten silicon-aluminum killed steel having a final composition consisting essentially of 0.04-O.30% of carbon, 0.20-1.50% of manganese, 0.02-0.15% of silicon, about 004-0. 10% of aluminum and the remainder iron and incidental impurities including nonmetallic inclusions, thereafter introducing the said molten silicon-aluminum killed steel into a mold for continuously casting steel, the molten siliconaluminum killed steel being introduced into the mold through a nozzle having a minimum internal diameter of 1% inches when the said molten silicon-aluminum killed steel contains 004% of aluminum and a minimum internal diameter of 1% inches when the said molten silicon-aluminum killed steel contains 0.10% of aluminum, the minimum internal diameter of the nozzle varying directly with the aluminum content in the steel within the foregoing ranges, the internal diameter of the said nozzle being sufficiently large to prevent plugging thereof by the nonmetallic inclusions, and casting the molten silicon-aluminum killed steel in the said mold to produce a solidified steel shape continuously, said solidified steel shape comprising 0.02-0.15% of silicon and about 0.040.10% of aluminum and when in the form of a slab being capable of being hot rolled and then cold rolled to produce flat rolled siliconaluminum killed sheet steel products characterized by good hardness, toughness, drawability, formability, stability and ductility.

2. The process of claim 1 wherein said molten siliconaluminum killed steel contains less than 0.04% of phosphorus, less than 0.05% of sulfur, and less than 125 parts per million of oxygen.

3. The process of claim 1 wherein said molten siliconaluminum killed steel contains about 0.060.08% of silicon, about 0.04-0.05% of aluminum, and less than parts per million of oxygen.

4. The process of claim 1 wherein said molten siliconaluminum killed steel contains about 0.060.08% of silicon, about 0.04-0.05% of aluminum, about 0005-0029? of phosphorus, about 0.0120.035% of sulfur, and less than 125 parts per million of oxygen.

5. The process of claim 1 wherein the molten siliconaluminum killed steel in said mold is cast into a solidified steel slab continuously, and the slab is hot rolled and then cold rolled to produce a flat rolled sheet steel product characterized by good hardness, toughness, drawability, formability, stability and ductility.

6. The process of claim 1 wherein the aluminum content of said molten silicon-aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.

7. The process of claim 1 wherein a fluid layer of slag covers the molten steel in the mold of said molten silicon-aluminum killed steel is introduced into the mold beneath the slag layer.

8. The process of claim 1 wherein said molten siliconaluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold.

9. The process of claim 1 wherein said molten siliconaluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold, a fluid layer of slag covers the molten steel in the mold, the molten steel is introduced into the mold beneath the slag layer, and the aluminum content of the molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified shape.

10. The process of claim 9 wherein said molten silicon-aluminum killed steel contains about 0.060.08% of silicon, about 0.040.05% of aluminum, about 0.0050.02% of phosphorus, about 0.0l20.035% of sulfur and less than 100 parts per million of oxygen, the molten killed steel in said mold is cast into a solidified steel slab continuously, and the slab is hot rolled and then cold rolled to produce steel strip characterized by good hardness, toughness, drawability, formability, stability and ductility.

11. The process of claim I wherein said heat of oxygen-containing carbon steel is prepared by a basic oxygen process and said heat is deoxidized by adding ferrosilicon and metallic aluminum thereto to produce a heat of molten silicon-aluminum killed steel containing less than 125 parts per million of oxygen.

12. The process of claim 11 wherein said heat of steel prepared by the basic oxygen process contains initially not more than 0.30% of carbon, less than 1.0% of manganese, less than 0.02% of silicon, less than 0.04% of phosphorus, less than 0.05% of sulfur, less than 800 parts per million of oxygen, and the remainder iron and incidental impurities.

13. The process of claim 12 wherein the aluminum content of said molten silicon-aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.

14. The process of claim 12 wherein a fluid layer of slag covers the molten steel in the mold and said molten silicon-aluminum killed steel is introduced into the mold beneath the slag layer.

15. The process of claim 12 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold.

16. The process of claim 12 wherein said molten silicon-aluminum killed steel contains about 0.060.08% of silicon and about 0.040.05% of aluminum.

17. The process of claim 12 wherein said heat of steel prepared by the basic oxygen process contains initially about 0.060.08% of carbon, about 0. l0-0.40% of manganese, less than 0.01% of silicon, less than 0.02% of phosphorus, less than 0.03% of sulfur, about 400-700 parts per million of oxygen, and the remain der iron and incidental impurities.

18. The process of claim 17 wherein said molten silicon-aluminum killed steel contains about 0.06 008% of silicon and about 0.040.05% of aluminum.

19. The process of claim 18 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold, a fluid layer of slag covers the molten steel in the mold and the molten steel is introduced into the mold beneath the slag layer, and the aluminum content ofthe molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.

20. The process of claim 19 wherein the said heat of molten silicon-aluminum killed steel contains at least 300 tons of steel and is introduced into said mold and cast into a solidified steel slab continuously without changing nozzles and the resulting solidified slab is cut into desired lengths and hot rolled and then cold rolled to produce steel strip characterized by good hardness, toughness drawability, formability, stability and ductil- 

2. The process of claim 1 wherein said molten silicon-aluminum killed steel contains less than 0.04% of phosphorus, less than 0.05% of sulfur, and less than 125 parts per million of oxygen.
 3. The process of claim 1 wherein said molten silicon-aluminum killed steel contains about 0.06-0.08% of silicon, about 0.04-0.05% of aluminum, and less than 100 parts per million of oxygen.
 4. The process of claim 1 wherein said molten silicon-aluminum killed steel contains about 0.06-0.08% of silicon, about 0.04-0.05% of aluminum, about 0.005-0.02% of phosphorus, about 0.012-0.035% of sulfur, and less than 125 parts per million of oxygen.
 5. The process of claim 1 wherein the molten silicon-aluminum killed steel in said mold is cast into a solidified steel slab continuously, and the slab is hot rolled and then cold rolled to produce a flat rolled sheet steel product characterized by good hardness, toughness, drawability, formability, stability and ductility.
 6. The process of claim 1 wherein the aluminum content of said molten silicon-aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.
 7. The process of claim 1 wherein a fluid layer of slag covers the molten steel in the mold of said molten silicon-aluminum killed steel is introduced into the mold beneath the slag layer.
 8. The process of claim 1 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold.
 9. The process of claim 1 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold, a fluid layer of slag covers the molten steel in the mold, the molten steel is introduced into the mold beneath the slag layer, and the aluminum content of the molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified shape.
 10. The process of claim 9 wherein said molten silicon-aluminum killed steel contains about 0.06-0.08% of silicon, about 0.04-0.05% of aluminum, about 0.005-0.02% of phosphorus, about 0.012-0.035% of sulfur and less than 100 parts per million of oxygen, the molten killed steel in said mold is cast into a solidified steel slab continuously, and the slab is hot rolled and then cold rolled to produce steel strip characterized by good hardness, toughness, drawability, formability, stability and ductility.
 11. The process of claim 1 wherein said heat of oxygen-containing carbon steel is prepared by a basic oxygen process and said heat is deoxidized by adding ferro-silicon and metallic aluminum thEreto to produce a heat of molten silicon-aluminum killed steel containing less than 125 parts per million of oxygen.
 12. The process of claim 11 wherein said heat of steel prepared by the basic oxygen process contains initially not more than 0.30% of carbon, less than 1.0% of manganese, less than 0.02% of silicon, less than 0.04% of phosphorus, less than 0.05% of sulfur, less than 800 parts per million of oxygen, and the remainder iron and incidental impurities.
 13. The process of claim 12 wherein the aluminum content of said molten silicon-aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.
 14. The process of claim 12 wherein a fluid layer of slag covers the molten steel in the mold and said molten silicon-aluminum killed steel is introduced into the mold beneath the slag layer.
 15. The process of claim 12 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold.
 16. The process of claim 12 wherein said molten silicon-aluminum killed steel contains about 0.06-0.08% of silicon and about 0.04-0.05% of aluminum.
 17. The process of claim 12 wherein said heat of steel prepared by the basic oxygen process contains initially about 0.06-0.08% of carbon, about 0.10-0.40% of manganese, less than 0.01% of silicon, less than 0.02% of phosphorus, less than 0.03% of sulfur, about 400-700 parts per million of oxygen, and the remainder iron and incidental impurities.
 18. The process of claim 17 wherein said molten silicon-aluminum killed steel contains about 0.06-0.08% of silicon and about 0.04-0.05% of aluminum.
 19. The process of claim 18 wherein said molten silicon-aluminum killed steel is agitated by passing an inert gas therein prior to introducing the steel into the mold, a fluid layer of slag covers the molten steel in the mold and the molten steel is introduced into the mold beneath the slag layer, and the aluminum content of the molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.
 20. The process of claim 19 wherein the said heat of molten silicon-aluminum killed steel contains at least 300 tons of steel and is introduced into said mold and cast into a solidified steel slab continuously without changing nozzles and the resulting solidified slab is cut into desired lengths and hot rolled and then cold rolled to produce steel strip characterized by good hardness, toughness drawability, formability, stability and ductility. 